A secondary battery that is safe and high in reliability in any terrestrial environment is requested for an environmental power generation technology by which electricity generated from microscopic energy such as sunlight, vibration or body temperature of humans or animals is accumulated and utilized for a sensor or as wireless transmission power.
As regards a liquid-based secondary battery used widely at present, it is a concern that, as a cycle of use is repeated, a positive electrode active material may degrade to decrease the battery capacity and the organic electrolyte in the battery may be ignited by battery short-circuiting caused by formation of dendrite.
Therefore, a secondary battery including a liquid-based electrolyte is poor in reliability and safety where it is intended to use the secondary battery for an environmental power generation device that is expected to be used, for example, for 10 years or more.
Therefore, attention is paid to an all-solid lithium secondary battery in which all constituent materials are solid. The all-solid lithium secondary battery is free from a risk of liquid leakage, firing or the like and is superior in a cycle characteristic.
For example, as solid electrolytes used for the all-solid lithium secondary battery, namely, as lithium ionic conductors, Li2S—B2S3 based (Li3BS3), Li2S—P2S5 based (Li7P3S11, Li3PS4, Li8P2S6 and so forth), Li2S—P2S5—X (LiI, B2S3, AI2S3, GeS2) based (Li4−xGe1−xPXS4), Li2S—B2S3—LiI based solid electrolytes and so forth are available. Also solid electrolytes having Li and S and having, as occasion demands, an element such as P, B or O (Li7P3S11, Li2S, Li3PO4—Li2S—B2S3 based, 80Li2S-20P2S5 solid electrolytes and so forth) are available.